I removed the front element of the Canon EF-S 17-55mm f/2.8 IS USM to clean a load of dust out of it. I thought there was already a thread discussing the dust issues with the Canon EF-S 17-55mm f/2.8 IS USM lens.

Any how, to my knowledge and benefit to get me going on this, the only place I know that shows some detail and dialogue about how to remove the front element on the Canon EF-S 17-55mm f/2.8 IS USM lens is over on pbase. I took that information and made a detailed video tutorial HERE.

I will say that the way the fellow mentioned the 3 screws were for front/back focus calibration is wrong. Simply by moving a screw you do not calibrate the lens for front/back focus. What he most likely encountered is rotating CCW or CW form its original postion. It's the rotating of the front element that is on an inclined resting plane that makes the lens shift further away or closer to the internal group elements. The reason there are 3 holes for each of the 3 screws is that as you move the element the slot to put the screws may cover up one of the holes and having two extra holes allows you to secure the lens. Kind of confusing but the video explains it better.

Hope it's helpful for you all.
-Roy

Roy, but I believe it is better to post this on the Canon Board since it is strictly Canon related.... Just a suggestion. Maybe one of the mods can transfer it over there.

1: Don't use a rocket blower, it will just blow dust further into the lens!


2:Pry the ring out with a toothpick? Sure, if you're dead-broke and want to take a chance on the toothpick breaking and sliding over / scratching your lens. If that ring is really a press-fit, and not a screw-on like most of them, then surely there's a more correc tool for the job.

I'm thinking there must be light threads, and this guy didn't notice (in which case, a set of rubber ring tools is cheap and correct) but maybe not. I'd like to know for sure.


3: I'm pretty sure those front screws are there to adjust infinity-focus stop, (like on most lenses) not front-back focus. According to a tech I once spoke with, the latter is an electronic adjustment on most AF lenses and not user-adjustable. Note that I could be wrong on this last point, but I doubt it. I'd be curious to here from someone who really knows - and if this guy is right, there where ARE the infinity adjust screws?

This guy may be a hack, pretending to be an expert. Tread lightly.

Cableaddict,

I’m going to assume by your statement “This guy may be a hack, pretending to be an expert.” That you are talking about the guy who made the video which in this case is me. I will agree with you, I’m not an expert. I hope I’m clear enough that this is a “do at your own risk”.

So to date, the only two web links I can find regarding this procedure are here: http://www.pbase.com/lightrules/drp and here: http://www.pbase.com/rcicala/1755_is_surgery I took this information and did it myself and made it much clearer in video format, only I didn’t use a metal item to remove the thin plastic trim-ring covering the screws and gaping slots. Yes, it’s just a thin plastic cover w/o threads of any kind and only held into place with a micro thin layer of sticky compound; which is why dust gets into the lens so easily. So your comment “I'm thinking there must be light threads, and this guy didn't notice” makes me think, did you even watch the video? I used a blunt toothpick and pried from the outer edge…the tooth pick never came close to the glass, if you call 5mm close.

Now for the rocket blower, I need to add an annotation and state this too. I used a staticly charged soft brush from copper hill images to lightly brush and collect the major dust, then I used my rocket blower. Bottom line is I’ve used the lens for the past 2 weeks since cleaning and it still doesn’t have any dust behind the element like it once had.

Now since I’m not an expert, I do know that just moving the screws from one of the 3 holes once they came will not adjust anything, like one of the links above say it does. I do know what rotating the lens cww/cw makes the element move up/down which changes something and in my tests it changed the “non infinity” focus points. Before I started with the cleaning I shot those 3 subjects in the video at f/2.8 in AF at about 2 feet at 17mm and about 10 feet at 55mm and it seemed that from the center focus point that it was back focusing a bit. Because I have rotated my front element about 1” cw, thus raising it, the same test results in move of a center focused capture.

Now like you, I too want to know if the front element is moved closer or further (which is what is taking place here) from the camera is this affecting AF focus point (aka front/back focus issues) OR is it changing the infinity focus stop?

I’m heading out to do some infinity focus tests.

*Update*
Well, I did some infinity focus tests...did some research and w/o a lab about all I could do is set the camera on a tripod with a cable release and take 10 shots all in MF starting at infinity, then backin off slightly. Then I took the same shot in AF and according to the distance scale on the lens it was way closer than infinity and I was focusing using center AF on a building about 3 miles away from the top of another building. Doing other hand-held tests it seems the AF is focusing according to the scale closer. Now looking at the AF shot and the infinity shot it seems the infinity manual shot looks sharper than the AF.

Bottom line and I reflected this in my video. Don't try to change anything...just use the marker tape and put the front element back exactly where it was before.

-Roy

Fair enough.

Thanks for the update. Glad you were able to think it through.